Viruses as Pathogens

• Viruses are obligate intracellular pathogens. – They are not living and cannot metabolize to create energy.– They cannot reproduce without the help of the host and its

resources. • Read up on TMV (Tobacco mosaic virus).

Structure• extremely small (can fit thousands on a

pinhead) – smallest 20nm (RBC 100mm, cardiac cell

1500mm, fat cell 6000mm)• nucleic acid

– single or double stranded DNA or RNA • protein coat

– may be a membranous envelope • derived from the membrane of the host • carry glycoproteins specific to the host cell • are usually animal viruses

– may be called a capsid • made of specialized proteins called capsomeres • rod or polyhedral shaped • mostly found in bacteria that infect bacteria-

bacteriophages

Viral infectionEntering a host - beginning of an infection • infection begins when viral nucleic acids enter a

host • T-phages inject DNA/RNA through their tail • envelope may fuse with host bringing in the nucleic

material via endocytosis

Viral infection

• Host range - defines who can be infected – can be narrow or broad • West Nile Virus - broad

(birds, humans, equine) • measles & poliovirus -

narrow(humans – is dictated by the

surface proteins present on the capsid/envelope

• Viral transmission– Horizontal - infection from an external source – Vertical - infection inherited from a parent (more common

in plants)

Types of infections• Lytic Cycle - virulent phage

– phage DNA enters cell (T4 through tail)

– cell's DNA is hydrolyzed (separated)

– Synthesis of viral DNA and proteins by host

– Assembly of complete virus – release as the cell swells and

bursts releasing many new complete viruses

Types of infections

• Lysogenic Cycle– replication without killing host – phage DNA enters cell

• incorporates itself into the hosts DNA – – now known as a prophage

• may lie within the host dormant and create many cells carrying the prohage DNA – a trigger can switch the prophage into a lytic cycle

– l phage is like the T4 but is not an obligate lysogenic virus - used in often research

Defense mechanisms• Host

– restrictions enzymes (endonucleases) • recognize viral DNA and cut it up

– evolution - favors host with different cell receptors

– vaccines • made of attenuated (viral pieces which are

harmless) viruses – medicines

• usually work by inhibiting viral DNA/RNA replication

• virus – mutation resistant to restriction enzymes – lysogenic lifecycle

Human Viruses• dsDNA

– Adenoviris (common cold)– Herpesviris (herpes, chicken

pox)– Poxvirus (smallpox, cowpox)

• ssRNA template for mRNA– Orthomyxovirus (influenza)

• ssRNA template for mRNA– Retrovirus (HIV)

Virus Types• Retrovirus: HIV• ssRNA virus equipped with the enzyme

reverse transcriptase – makes DNA from RNA - reverse process – DNA is inserted into host's DNA - HIV

• now called a provirus – RNA pol II from the host now makes mRNA

capable of making more viral ssRNA – provirus NEVER leaves the host

Virus Types• Viroids and Prions

– Viroids - circular pieces of RNA that infect plants • smaller than viruses • Caused by the misfolding of proteins• do not encode proteins but take over the regulatory system of its host cell causing

mass replication – Prions

• small viral pieces that cause diseases in animals • cause neural diseases - mad cow (BSE), scrapie (sheep) • slow onset time • cannot be killed by heating and cooking • no known cure